﻿//https://leetcode.cn/problems/validate-binary-search-tree/?envType=study-plan-v2&envId=top-100-liked

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    long prev = LONG_MIN;
public:
    bool isValidBST(TreeNode* root)
    {
        if (root == nullptr) return true;

        bool left = isValidBST(root->left);
        if (left == false) return false;  //剪枝

        bool cur = false;
        if (root->val > prev) { cur = true; prev = root->val; }
        else return false; //剪枝

        bool right = isValidBST(root->right);

        return left && right && cur;

    }
};